As a method of amplifying a signal without using an enzyme, Patent Document 1 discloses a method of amplifying a signal (hereinafter referred to as a PALSAR method), in which two sets of dimer probes, that is, a dimer probe having a structure of the following formula (1) and a dimer probe having a structure of the following formula (2) or (3), are used and an assembly substance (polymer) is formed by a self-assembly reaction of the dimer probes.

In the above formulae (1) to (3), a region A and a region A′, a region B and a region B′, a region C and a region C′, a region D and a region D′, a region E and a region E′, and a region F and a region F′ have complementary base sequences, respectively, and a self-assembly substance having a structure represented by the following formula (4) is formed by binding multiple dimer probes. The signal probe polymer as used, herein refers to a nucleic acid probe polymer including the self-assembly substance formed of the dimer probes.

Patent Document 2 discloses a method of detecting a gene to be tested in a sample by using the above PALSAR method. Patent Document 2 discloses a method of detecting a gene to be tested with a high sensitivity by forming a complex of the gene to be tested and a probe polymer and detecting the probe polymer, and discloses a method of designing so that a part of one of multiple kinds of probes used for forming a polymer has a base sequence complementary to that of the gene to be tested, and a method of using a probe (an assist probe) having, at different site, both a base sequence complementary to that of the gene to be tested and a base sequence complementary to that of a probe used for forming a polymer as methods of forming a signal probe polymer on the gene to be tested. Note that the assist probe as used herein refers to a probe having both a region capable of binding to a target substance to be detected and a sequence complementary to that of a probe used for forming a polymer, and serves to connect the target substance with a signal probe polymer. A method of using the assist probe has an advantage in that multiple genes may be simultaneously detected with the same set of multiple kinds of probes by preparing multiple assist probes whose region complementary to a gene to be tested are different from each other.
Patent Document 2 describes an assist probe having a sequence complementary to one region of a dimer probe, but does not mention any study on an assist probe capable of detecting a target substance with high sensitivity.    Patent Document 1: WO 02/031192    Patent Document 2: WO 2006/028162